This invention relates to the decommissioning of nuclear reactors which are considered to have completed their operational life, and in particular, to machines for effecting dismantling of the reactor structure in a controlled and safe manner.
When a nuclear reactor has reached the end of its planned operational life, the alternatives, after removal of the nuclear fuel and reactor coolant, are to cover the whole structure with adequate shielding, such as a mound of soil, and take precautions for the covered reactor to be preserved so shielded for all time as a permanent `memorial`, or to dismantle the structure of the reactor and dispose of the dismantled material in small amounts in conventional safe storage facilities, and as a result, restore the reactor site to its pre-building state. The latter course is to be preferred, especially in view of doubts concerning the practicability of preserving the shielding integrity of the whole reactor structure over the period of thousands of years necessary before biologically unsafe amounts radioactivity will have decayed away.
To put the present invention into context, consider a conventional gas-cooled, graphite moderated, nuclear reactor which has within a pressure vessel a core including the moderator and the fuel elements, the latter being capable of being charged and discharged by a refuelling machine, and surrounding the core, heat exchangers which remove heat from coolant heated by circulation through the core and employ the removed heat to generate electricity, for example employing steam and a turbine. When such a reactor has completed its operational lifetime, let it be assumed that a decision has been reached to dismantle the reactor and restore the site. After normal discharge of the fuel elements and blow-down of the coolant gas it is considered acceptable from a safety point of view to employ conventional dismantling techniques for removal of the turbines, heat exchangers, refuelling machine and, with adequate temporary shielding, the top dome of the pressure vessel so as to expose the core. In order to dismantle and remove the pressure vessel and to unload and transfer to to disposal facility the graphite moderator (which is in discrete block form) and other vessel internals, a machine is required which will perform these operations safely and expeditiously, and it is an object of the present invention to provide such a machine.